What have been actively carried out in recent years are researches and developments for a current driving light-emitting element such as an organic EL display and an FED. Especially, the organic EL display is such a display that can emit light by a low voltage and less power consumption, so that the organic EL display has drawn attention as a display for a mobile device such as a mobile phone or a PDA (Personal Digital Assistants).
As a structure example of a current driving pixel circuit of such an organic EL display, FIG. 22 illustrates a circuit structure described in “Active Matrix PolyLED Displays” (M. T. Johnson et al., IDW '00, 2000, pp. 235-238) and WO 99/65011 (published Dec. 16, 1999).
In the circuit structure shown in FIG. 22, a driving TFT (Thin Film Transistor) 101 has a source terminal connected to a power source wire Vs, and has a gate terminal connected to the power source wire Vs via a capacitor 104. Moreover, a switching TFT 102 is provided between a drain terminal of the driving TFT 101 and an anode of an organic EL element 103. A cathode of the organic EL element 103 is connected to a common wire Vcom.
Further, a selection TFT 106 and a switching TFT 105 are provided at a node of the driving TFT 101 and the switching TFT 102. The selection TFT 106 has a source terminal connected to a source wire Sj. The switching TFT 105 has a source terminal connected to the gate terminal of the driving TFT 101.
In the structure, when a signal indicative of Low is supplied to a scan wire Gi (selection period), the switching TFT 102 becomes OFF, and the selection TFT 106 and the switching TFT element 105 becomes ON. In this case, a current can flow from the power source wire Vs to the source wire Sj via the driving TFT 101 and the selection TFT 106. In cases where the current thus flowing is controlled by a power source of a source driver circuit (not shown) connected to the source wire Sj, a gate voltage of the driving TFT element 101 is set such that a current specified by the source driver circuit flows into the driving TFT 101.
On the other hand, when the scan wire Gi receives a signal indicative of High (non-selection period), the selection TFT 106 and the switching TFT 105 become OFF, and the switching TFT 102 becomes ON. During this non-selection period, the capacitor 104 retains the gate potential, determined during the selection period, of the driving TFT element 101. With this, during the non-selection period, the driving TFT 101 allows the determined current to flow into the organic EL element 103.
As an example of a current driving pixel circuit structure similar to the above structure, FIG. 23 illustrates a pixel circuit structure described in “Polysilicon TFT Drivers for Light Emitting Polymer Displays” (Simon W-B. Tam et al., IDW '99, 1999, pp. 175-178) and WO 98/48403 (published on Oct. 29, 1998).
In the circuit structure shown in FIG. 23, a capacitor 111 is provided between a source terminal of a driving TFT 108 and a gate terminal thereof. Between the gate terminal of the driving TFT 108 and a drain terminal thereof, a switching TFT 112 is provided. The drain terminal of the driving TFT 108 is connected to an anode of an organic EL element 109. Further, a switching TFT 107 is provided between the source terminal of the driving TFT 108 and a power source wire Vs. Moreover, a selection TFT 110 is provided between the source terminal of the driving TFT 108 and a source wire Sj.
The selection TFT 110 has a gate terminal connected to a control wire Wi, and the switching TFT 107 has a gate terminal connected to a control wire Ri, and the switching TFT 112 has a gate terminal connected to a scan wire Gi.
The following explains an operation of the pixel circuit structure with reference to a timing chart shown in FIG. 24. The timing chart illustrates respective timings of supplying signals to the control wires Wi and Ri, the scan wire Gi, and the source wire Sj.
In FIG. 24, the selection period corresponds to a period of time from 0 to 3t1. During the selection period, the control wire Ri has a High (GH) potential, so that the switching TFT 107 is OFF. On this occasion, the control wire Wi has a Low (GL) potential, so that the selection TFT 110 is ON. With this, during the selection period, a current flows from the source wire Sj to the organic EL element 109 via the selection TFT 110 and the driving TFT 108.
During a period of time from 0 to 2t1 within the selection period, the scan wire Gi has a High potential, so that the switching TFT 112 is ON. Accordingly, a current flows from (i) a source driver circuit (not shown) connected to the source wire Sj, to (ii) the organic EL element 109. This determines a gate potential of the driving TFT 108 such that a current specified by the source driver circuit flows into the organic EL element 109. During a period of time from 2t1 to 3t1, the switching TFT 112 is OFF; however, the gate potential of the driving TFT 108 is retained by the capacitor 111. This allows a current to flow from the source wire Sj to the organic EL element 109 during this period, too.
After the time 3t1 (non-selection period), the switching TFT 110 is OFF, and the switching TFT 107 is ON. With this, during the non-selection period, a determined current is controlled to flow from the power source wire Vs to the organic EL element 109.
However, the following problem arises in the pixel circuit structure described in “Polysilicon TFT Drivers for Light Emitting Polymer Displays” (IDW '99, pp. 175-178). That is, variation in a threshold voltage of the driving TFT 108, and variation in mobility thereof cause variation in the current flowing into the organic EL element 109 during the non-selection period.
For the purpose of clarifying an adverse effect of such variation of the current, a simulation was carried out under the following five conditions shown in Table 1 below. The simulation found respective values of currents flowing to the organic EL element 109. The simulation result is shown in FIG. 25.
TABLE 1Ioled(1)Ioled(2)Ioled(3)Ioled(4)Ioled(5)ThresholdAverageLowerUpperUpperLowervoltagevaluelimitlimitlimitlimitvaluevaluevaluevalueMobilityAverageLowerUpperLowerUppervaluelimitlimitlimitlimitvaluevaluevaluevalue
In the simulation shown in FIG. 25, the selection period came every 0.24 ms. During an initial period of time from 0.27 ms to 0.51 ms, a current of 0.1 μA flowed to the source wire Sj. Thereafter, the current was increased by 0.1 μA every 0.24 ms until the current had a value of 0.9 μA. Then, the current was set at 0. After that, the current was increased again by 0.1 μA.
Specifically, a first selection period corresponds to a period of time from 0.27 ms to 0.30 ms. The current, which had a value of 0.1 μA and which was flowing to the source wire Sj during the selection period, determined the potential of the gate terminal of the driving TFT 108. This determined that the current of 0.1 μA flowed to the organic EL element 109 only during the selection period. Note that the gate potential on this occasion was retained during a following non-selection period continuing from 0.31 ms to 0.51 ms; however, the current flowing to the organic EL element 109 during the non-selection period varied in a range from 0.12 μA to 0.13 μA.
The variation found by the simulation is illustrated in FIG. 26 whose horizontal axis plots the respective currents (the respective ten currents of 0 to 0.9 μA) which flowed to the source wire Sj, and whose vertical axis plots respective currents which flowed to the organic EL element 109 during the non-selection periods, each of which came after each current supply to the source wire Sj. In FIG. 26, during the non-selection period coming after the supply of the current of 0.9 μA to the source wire Sj, the current that flowed through the organic EL element 109 varied in a range of about 0.95 μA to about 1.12 μA (increases by 5% to 24%).
Such variation is caused by a difference between (i) a source-drain voltage Vsd of the driving TFT 108 during the selection period (period of time from approximately 270 μs to approximately 300 μs), and (ii) a source-voltage Vsd during the non-selection period (period other than the selection period), as shown in FIG. 27. Note that FIG. 27 illustrates a result of a simulation carried out under the five conditions (see Table 2) of the threshold voltage and the mobility of the driving TFT 108. Note also that voltage values Vsg (1) through Vsg (5) respectively correspond to the conditions Ioled (1) through Ioled (5) (see Table 2), and that voltage values Vsd (1) through Vsg (5) respectively correspond to the conditions Ioled (1) through Ioled (5).
Namely, in the circuit structure shown in FIG. 23, the switching TFT 112 was ON upon the current writing (period of time from 0 to 2t1 in FIG. 24; period of time from about 270 μs to about 290 μs in FIG. 27) carried out during the selection period, so that each source-drain voltage Vsd coincided with each source-gate voltage Vsg, as shown in FIG. 27.
The source-gate potential Vsg that the driving TFT 108 had on this occasion was determined according to the threshold voltage of the driving TFT 108 and the mobility thereof. Specifically, comparing (i) a case where the threshold voltage is 1 V with (ii) a case where the threshold voltage is 2V, the source-gate voltages Vsg varies by on the order of 1V. In fact, the above simulation result shows that the source-gate voltage Vsg varied in a range from about 1.4 V to about 3.6 V when the current of 0.1 μA was supplied to the source wire Sj.
Thereafter, when the switching TFT 112 was turned OFF (at about 290 μs), the source-gate potential of the driving TFT 108 was retained; however, the source-drain voltage Vsd thereof was changed.
Especially after the pixel circuit was brought into the non-selection period (at about 300 μs), the source-drain voltage Vsd was changed to be approximately 6 V. The voltage Vsd is determined according to “applied-voltage/current property” of the organic EL element 109. The wording “applied-voltage/current property” refers to a property indicating a relation between the applied voltage and the current. In other words, the voltage Vsd is determined according to a voltage Voled required for the supply of the current of 0.1 μA to the organic EL element 109. In the simulation, the voltage Voled had such a property as to satisfy:Voled=Vs−6VFurther, the applied-voltage/current property of the organic EL element 109 is similar to a property of a diode (the current exponentially increases in response to the applied voltage). For this reason, even when the current flowing through the organic EL element 109 varies by several ten percent, the source-drain voltage of the driving TFT 108 does not vary greatly.
If the driving TFT 108 were an ideal TFT, the change of the source-drain voltage would never cause the change of the current flowing from the source terminal of the driving TFT 108 to the drain terminal thereof, in cases where the gate-source potential Vsg is constant, and where the source-drain voltage Vsd is larger than the gate-source potential Vsg. However, in an actual TFT, even in cases where a gate-source potential Vsg is constant, a current flowing from a source terminal of the TFT to a drain terminal thereof increases as a source-drain voltage Vsd increases, as shown in FIG. 28. Note that FIG. 28 illustrates a result of a simulation carried out under the five conditions (see Table 2) of the threshold and the mobility of the driving TFT 108. Note also that current values Itft (1) through Itft (5) respectively correspond to the conditions Ioled (1) through Ioled (5) (see Table 2).
The result shown in FIG. 28 indicates that the variation of the source-drain voltage Vsd upon the current writing causes the variation of the current flowing from the source to the drain during the non-selection period. This changes the current flowing through the organic EL element 109.
So, an examination was carried out in order to find such current variation between the source terminal of the driving TFT 108 and the drain terminal thereof, with the use of a circuit in which the driving TFT 108 and the organic EL element 109 are provided in series as shown in FIG. 29. The examination is carried out by simulating a current flowing through the organic EL element 109, in the following manner under the aforesaid five conditions of the threshold voltage and the mobility of the driving TFT 108. That is, the simulation is carried out by (i) applying, to the gate terminal of the driving TFT 108, the gate-source potential Vgd, obtained upon the current writing (see FIG. 27), of the driving TFT 108; and (ii) changing a power source voltage Vs−Vcom. A result of the simulation is shown in FIG. 30.
FIG. 30 shows a case of the gate-source potential Vgd upon the supply of a current of 0.5 μA to the source wire Sj. In this case, the potential of the source wire Sj upon the current writing (See FIG. 27) was changed according to each of the conditions of the threshold voltage of the driving TFT 108 and the mobility thereof. This determined that the current of 0.5 μA was supplied to the organic EL element 109. Therefore, the current flowing through the organic EL element 109 was changed, on condition that the potential of the power wire Vs is constant (16V).
In this way, the threshold voltage variation of the driving TFT, and the mobility variation thereof cause the variation of the source-drain voltage Vsd upon the current writing, with the result that the current flowing through the organic EL element varies during the non-selection period. Such a phenomenon occurs also in the pixel circuit structure shown in FIG. 22. As such, the conventional circuit structure suffers from such a problem that the threshold voltage variation of the driving TFT and the mobility variation thereof cause the variation of the current flowing through the organic EL element during the non-selection period.
The present invention is made to solve the problem, and its object is to provide a display apparatus that is able to restrain the variation of the current flowing through the organic EL element during the non-selection period, the variation of the current being caused by the threshold voltage variation of the driving TFT, and the mobility variation thereof.